Electro-static contaminant removal from light modulating fluid

ABSTRACT

In the space between the opposed output window and rotating disk of a light valve, loose particulate material in the fluid which becomes charged and held on the nonconductive surface of the disk or the inside surface of the output window is repelled from one surface to the opposing surface by applying a high voltage between the disk and exterior conductive shell. By periodically reversing polarity of the applied voltage, with an interval of zero voltage interposed between opposite polarity intervals, the particles are moved back and forth between the disk and output window. When the particles adhere to the disk, they are transported toward the fluid reservoir where they are repelled from the disk into the bulk of the fluid and are subsequently pulled through the fluid filter.

United Sta Towlson Inventor: Howard E. Towlson, Baldwinsville,

Apr. 1, 1975 [75 l N Y 57 ABSTRACT v [n the space between the opposedoutput window and [73] Asslgnee: General Elecmc Company rotating disk ofa light valve, loose particulate material surmise in the fluid whichbecomes charged and held onthe 23 Filed; Sept 10 1973 nonconductivesurface of the disk or the inside surface of the output window isrepelled from one surface to [Zn Appl' N05 395514 the opposing surfaceby applying a high voltage between the disk and exterior conductiveshell. By peri- 1521 us. or .1 350/101, 204/299, 204/302, Odiwllyreversing P y of the pp voltage, with 3 5 an interval of zero voltageinterposed between oppo- 51 Int. Cl. G02f 1/32, BOld 13/02 Site Polarityintervals, the Particles are moved back 1581 Field of Search, 350/101;204/299, 302; and forth between the disk and Output windowwhen 3 3 5 theparticles adhere to the disk, they are transported toward the fluidreservoir where they are repelled 5 References Cited from the disk intothe bulk of the fluid and are subse- UNITED STATES PATENTS quentlypulled through the fluid filter. 3.4219940 1/1970 Towlson 350/161 11Claims, 6 Drawing Figures FIG! 22 r s" Q 1 19* 26 25 3o-- l 14 gELECTRON eu- 5a f :3 52- f [.TEIIIEU APR I I mean FIG.4

TIME (MINUTES) FIGZ) j l I I l I I SYNCHRONOUS MOTOR ELECTRO-STATICCONTAMINANT REMOVAL FROM LIGHT MODULATING FLUID INTRODUCTION Thisinvention relates to light valves for optically projecting imagesgenerated electronically on a fluid layer, and more particularly to amethod and apparatus for electrostatically removing particulate materialadhering to the nonconductive surface of the disk or the inside surfaceof the output window and directing it through a filter for the fluid.

One form of light valve suitable for optical projection ofelectronically generated images onto a remote display surface comprisesan evacuated envelope containing an electron gun in alignment with atransparent disk. The disk is rotated through a reservoir oflightmodulating fluid to deposit a continuously replenished layer offluid on the disk surface. An electron beam, generated by the electrongun, is directed through electrostatic beam deflecting and focusingmeans and is scanned across a portion of the light-modulating fluidlayer so as to selectively deform the layer. The fluid deformations thusformed constitute diffraction gratings which, in conjunction with aSchlieren optical system, selectively control passage of light from alight source through the disk and through an output window in the lightvalve envelope in order to create visible images at a remote displaysurface on which the light impinges.

Light valves of the type described have hitherto operated satisfactorilyfor varying periods of time. Although the light-modulating fluid is freefrom particulate contamination at the outset of light valve operation, abuildup of contamination usually occurs as operation of the light valveprogresses. This contamination is due mainly to wear of moving partswithin the light valve, as well as presence of impurities. In addition,some particles are formed as a result of massive damage to fluidmolecules caused by heavy electron or ion bombardment. In accordancewith the invention shown and described in US. Pat. No. 3,489,940, issuedJan. 13, 1970 to H. E. Towlson and assigned to the instant assignee, thecontaminants may be isolated by use of a baffle plate spaced in closeproximity to the disk so that fresh filtered fluid dispensed on the sideof the plate facing the disk maintains a fluid buffer region around thearea of the disk whereon the fluid layer is deformed by the electronbeam. The buffer region contained between the plate and the diskprevents contaminated, particulate-carrying sump fluid from reaching thedisk surface facing the buffer plate.

Nevertheless, some contaminants do find their way onto the surface ofthe disk facing the output window and the inside surface of the outputwindow itself. As the particulate matter builds up on these opposingglass surfaces, performance of the light-valve can deteriorate to thepoint where, ultimately, the disk and window must be cleaned in orderfor satisfactory operation to take place. This involves a costlyprocedure inasmuch as the evacuated envelope must be opened, and hencethe light valve is essentially rebuilt.

The present invention concerns a light valve in which the nonconductive,front surface of the disk and the nonconductive, rear surface of theoutput window may be electrostatically cleaned at any time when thelight valve is not being used to display images. The electrostaticcleaning does not require the envelope of the light valve to be opened.Instead, an electric field of predetermined configuration is establishedacross a space encompassing the entire width of both the disk and outputwindow. The field, being of predetermined amplitude and polarity, andbeing cycled in a predetermined fashion, causes particles adhering, byelectrostatic attraction, to the front surface of the disk and to therear or inside surface of the output window, to be repelled from thesurface to which they are adherent, to the opposing glass surface. Thenext reversal of polarity causes these particles to transfer back to thesurface from which they originated, for example, particles from the diskthat had been repelled to the output window surface stick to the outputwindow until polarity is reversed, and are then transferred back to thedisk surface where they are carried along with the rotating disk. Inthis fashion, the particles are kept in motion in the fluid between thedisk and output window surfaces. By having a discontinuity in the lowerportion of the output window, particles repelled from the front surfaceof the disk are forced deep into the particulatefree fluid in the sumpand are subsequently pulled through the fluid filter.

Accordingly, one object of the invention is to provide a system forelectrostatically cleaning the interior surface of the output window andthe front surface of the rotating disk in a light valve. I

Another object is to provide a light valve wherein any deterioration inperformance due to particulate accumulation on opposing glass surfacesof the output window and rotating disk is reversible.

Another object is to provide electrostatic apparatus for preventing anysignificant quantities of contaminants in the fluid between the outputwindow and rotating disk from becoming electrostatically adhered toeither of these surfaces.

Another object is to provide a method of removing particulate matterfrom a portion of relatively stationary fluid situated within a narrowconfinement in communication with a larger quantity of such fluid.

Briefly, in accordance with a preferred embodiment of the invention, alight valve contains a rotatable disk on which a layer oflight-modulating fluid is carried, and an output window. A portion ofthe disk and output window are each submerged in a reservoir of thefluid, with a portion of the fluid being contained between the disk andoutput window. Contaminants are removed from the fluid by applying a dcvoltage of cyclically reversing polarity across the fluid between thedisk and output window while the disk is rotating, the voltage being ofsufficiently high amplitude electrostatically to move particulate matterthrough the fluid substantially normal to opposed disk and output windowsurfaces. Means are provided for withdrawing fluid containingparticulate matter from the reservoir and replenishing the reservoirwith fluid in which the content of particulate matter has beenminimized.

. In accordance with another preferred embodiment of the invention, amethod of removing particulate matter from a portion of fluid situatedbetween spaced surfaces located within a reservoir of the fluid, one ofthe surfaces being the surface of a rotating disk, comprises moving theparticulate matter back and forth between the surfaces to allowdifferent portions of the particulate matter to become attracted to therotating disk and be transported to a fluid outlet within the reservoir.Fluid containing the particulate matter in suspension is drawn off fromthe reservoir, and the drawn-off fluid is replaced in the reservoir withfluid in which the content of particulate matter is at a minimum.

BRIEF DESCRIPTION OF THE DRAWINGS The features of the invention believedto be novel are set forth with particularity in the appended claims. Theinvention itself, however, both as to organization and method ofoperation, together with further objects and advantages thereof, maybest be understood by reference to the following description taken inconjunction with the accompanying drawings in which:

FIG. 1 is a partially cutaway side view of a portion of a light valveemploying the instant invention;

FIG. 2 is a sectional view of the light valve shown in FIG. 1, takenalong line 2-2;

FIG. 3 is a schematic diagram showing means for applying anelectrostatic potential to a light valve for removing contaminants fromthe fluid,

FIG. 4 is an illustration of waveforms employed in electrostaticallyremoving contaminants from the fluid of a light valve;

FIG. 5 is a schematic diagram of apparatus for electromechanicallyproducing the waveforms shown in FIG. 4; and

FIG. 6 is a schematic and partially cutaway side view of apparatus forproducing the waveforms shown in FIG. 4.

DESCRIPTION OF TYPICAL EMBODIMENTS FIG. 1 illustrates a light valveincluding the electrostatic contaminant removal means of the instantinvention. The light valve comprises an envelope 10, typically of glass,containing a light output window portion 11 and a sump region 12 actingas a reservoir of lightmodulating fluid 13. The interior of enevelope isevacuated to a low gas pressure.

The light-modulating fluid is typically of the polybenzyltoluene typehaving a fluid viscosity of 1000 centistokes at 60C, with a vaporpressure in the range of l0 -l0 torr. The fluid contained in sump region12 is that which has drained off of an optically transparent disk 14which is continuously rotated on metallic bearings 15 about a metallicshaft 16, typically at a speed in the order of 3 revolutions per hour. Ametallic spring is maintained in compression by having its metallic cap21 affixed to a rigid, electrically conductive support member 22 which,in turn, is affixed to envelope 10 of the light valve by any suitablemeans (not shown). The opposite end of spring 20 bears against the bodyof a shaft 16. Consequently, a shoulder 23 on shaft 16 urges bearing 15to force disk 14 against a plurality of protuberances 17, which mayadvantageously be formed of fritted glass droplets. These protuberancesare affixed to output window 11.

Protuberances 17 maintain disk 14 at a distance of about 3 mils fromlight output window 11 so as to permit fluid 18 from reservoir 12 torise by capillary action and fill the region between the disk and theoutput window, as described in greater detail in H. E. Towlson U.S. Pat.No. 3,385,991, issued May 28, 1968 and assigned to the instant assignee.

While the front surface of rotating disk 14 is uncoated, a thin film 24,which comprises a transparent conductive coating such as indium oxide,is carried on the rear surface of the disk. Coating 24 may be maintainedat any desired potential since a conductive path is formed throughbearing 15, shaft 16, spring 20, cap

21 and member 22, permitting a continuous electrical connection tocoating 24 through a stationary connection (not shown) which may be madeto member 22. An aperture 19 in member 22 permits passage of an electronbeam 25, originating at an electron gun 26, to be directed towardsconductive coating 24 on disk 14. Disk 14 itself is nonconductive, andpreferably is comprised of glass.

A thin film of light-modulating fluid 27 is coated on conductive coating14 and thus is situated in the direct path of electrons in electron beam25. Beam 25 is focused and deflected by electron optical means (notshown) within the light valve and hence is swept, in raster fashion,over the surface of light-modulating fluid layer 27. The pattern ofcharges on layer 27 produced by electron beam 25 causes correspondingdeformations in the thickness of of layer 27, resulting in formation ofdiffraction gratings 30. These gratings correspond to the image to beprojected onto a remote display surface. Light from a light source (notshown) positioned behind electron gun 26 impinges upon a lenticular lenssystem 28 formed on the rear wall of envelope 10 and is directed by thelenticular lens system through aperture 19 onto diffraction gratings 30.

By modulation of electron beam 25 through application of suitablepotentials to the electrostatic focus and' deflection means, diffractiongratings 30 in fluid layer 27 are selectively controlled. Consequently,light passing through transparent rotatable disk 14 and output window 11is selectively controlled and, in conjunction with externally locatedlenses of a Schlieren optical system (not shown) is projected on aremote display surface (not shown) to fonn an image representative ofthe intelligence modulating the electron beam.

A baffle plate 40, situated within sump 12, is spaced in close proximityto, and parallel to, disk 14. The spacing and orientation of baffleplate 40 with respect to disk 14 are maintained by spacer bars 41. Byemploying a metallic baffle plate, bars 41 may be welded to the baffleplate and mated with cavities 42 in output window 11. The baffle plateis then rigidly held in place by two spring clips 43.

The manner by which spring clips 43 hold baffle plate 40 to outputwindow 11 is further illustrated in FIG. 2, which is a sectional viewtaken along line 2-2 of FIG. 1. In FIG. 2, the width of baffle plate 40can be seen to be larger than that of disk 14, while output window 11can be seen to have protruding regions 47 which position bars 41 andclips 43 at locations to avoid interference with rotation of disk 14.Baffle plate 40 may alternatively be constructed of glass, in which casespacer bars 41 are likewise constructed of glass and joined by frittingto output window 11 so as to render spring clips 43 unnecessary. Theseparation maintained between baffle plate 40 and disk 14 is typicallyin the order of 10-15 mils.

A raster area 48 on oil film 27, which is the region upon which electronbeam 25 may impinge to form diffraction gratings 30, is illustrated inits relative position on disk 14 in FIG. 2. Although raster area 48 isshown as a discrete area, and is fixed with respect to output window 11,those skilled in the art will recognize that the raster area on disk 14constitutes a circular band, due to the continuous rotation of the disk.

As shown in FIG. 1, the entire front end of the light valve is enclosedwithin a protective, metallic housing containing an opening 49 to allowegress of light.

The housing, typically comprised of aluminum, may conveniently be formedof two parts, a so-called clamshell 51 and support plate 52, securedtogether by bolts 53. The housing is supported upon glass protuberancesformed on the light valve, clamshell 51 being supported upon threeprotuberances 54 substantially equally spaced about the light valve sothat only one is illustrated, and support plate 52 being situated uponthree protuberances 55 also substantially equally spaced about the lightvalve so that only one is illustrated. To protect protuberances 54 and55 from damage due to forces exerted by housing 50, each ofprotuberances 54 is mated with a spring 56 fitted within a flanged cup57 recessed in clamshell 51, while each of protuberances 55 is matedwith a fiberglass pad 58 recessed in support plate 52.

Fluid conveying means 44 is disposed in predetermined configuration onthe side of baffle plate 40 facing disk 14. This configuration permitsflow of fresh filtered fluid outward in all directions parallel to theplane of disk 14, so as to maintain the entire region between disk 14and baffle plate 40 filled with fresh filtered fluid only. Bycontinually supplying fresh filtered fluid from a pump and filter 45,shown in FIG. 1, through a closed tube 46 to fluid conveying means 44,fluid pressure in the region between baffle plate 40 and disk 14 ismaintained sufficiently high to prevent contaminated fluid from sump 12from entering this region. Accordingly, as disk 14 rotates, transparentcoating 24 is continually covered with fresh filtered fluid only, sincethe surface of coating 24 does not contact any of the contaminated fluidin sump 12.

Despite the fact that fresh filtered fluid is coated upon the disk,contaminants in the fluid can still produce blemishes in the displayedimage if they arepresent in the region between light output window 11and disk 14 in an area aligned with electron gun 26 and any location inraster area 48 shown in FIG. 2. In this region, precautions arenecessary to insure cleanliness during assembly to minimize presence ofany objectionable contaminants in the light valve. Nevertheless, after afinite total time of operation, some contaminant particles may appear inthe region shown in FIG. 1 between light output window 1 1 and disk 14,and produce visible blemishes, typically doughnut-shaped spots, in thedisplayed image. Because of high intensity electrical fields that occurduring normal operation of the light valve, these contaminants becomecharged and consequently adhere, by electrostatic attraction, to theuncoated, nonconductive surface of disk 14 or the inside surface ofnonconductive output window 11. When the high intensity electricalfields are collapsed, however, as when the light valve is in a standbycondition as defined, infra, the contaminant particles slowly discharge.Nevertheless, these particles never move very far from the surfaces towhich they have adhered, since relative velocity of the fluid normal tothe surfaces of disk 14 and output window 11 within the region definedby these surfaces is essentially zero. Only those contaminant particlesthat adhere to the disk and are carried into reservoir 12 encounterfluid that is in motion and hence, if the light valve is switched into astandby condition at the time such particles are being transported intoreservoir 12 by the disk, they may drift away from the disk andultimately be removed by filtration through pump and filter 45. At best,this but slightly alleviates the problem created by such contaminantparticles since contaminant particles adhered to the output window andto other locations on the disk surface that are released into the regionbetween the disk and output window above the reservoir encounteressentially no moving fluid capable of transporting them to pump andfilter 45. When the light valve is next returned to an operatingcondition, the unremoved par ticles are once again attracted to thenonconductive surfaces of disk 14 and output window 11 through pathsnormal to the surfaces thereof.

By subjecting particles between disk 14 and output window 11 toelectrical fields that cause them to move back and forth in that regionwhile disk 14 is in rotation, disk 14 may be utilized as a means totransport contaminant particles from output window 11 as well as fromthe disk, into sump 12 for removal. As illustrated in FIG. 3, this isaccomplished by applying a high voltage dc source across a pair ofelectrodes shown in fragmentary form, one electrode comprisingconductive coating 24 on disk 14 and the other electrode comprisinghousing 50, so as to subject window 11, disk 14 and fluid 13 between thewindow and disk, to a high intensity electrical field. To avoiddisruption of normal light valve operation, contaminant removal isperformed when the light valve is in standby operation; that is, theelectron beam is turned off, as are its focus, deflection andacceleration fields, but disk 14 is rotated and pump and filter 45,shown in FIG. 1, are in operation. Depending upon relative polarities ofthe electrical fields and each contaminant particle, each particle isinfluenced to move toward either disk 14 or output window 11. Duringintervening periods of zero electrical field, the particles are free todrift away from the surfaces to which they had been attracted.Accordingly, by judicous selection of duty cycle for the electricalfield, the particles may all eventually be transported, by rotation ofdisk 14, into sump 12 for removal.

A typical duty cycle for voltage supplied by dc source of FIG. 3 isillustrated in FIG. 4. Thus, output voltage of the dc source is on fortwo and one-half minute intervals, switching from +8000 volts to -8000volts and back again to +8000 volts, with intervening zero voltageintervals of one-half minute each. During each positive voltageinterval, positive voltage is applied to conductive layer 24 on disk 14and negative voltage to housing 50, shown in FIG. 3, causingpositivelycharged contaminant particles to migrate through fluid 13toward output window 11. At the same time, negatively-charged particlesare caused to migrate through fluid 13 toward disk 14, and the motion ofthe disk carries these particles along until the voltage drops to zero,after which time the particles move into fluid 13. Thosenegatively-charged contaminant particles released from the disk in sump12, shown in FIG. 1, especially those released below the bottom ofoutput window 11, are caught up in the motion of fluid in the sump, andare filtered by pump and filter 45 so as to be removed from the fluid.Those contaminant particles released into fluid 13 between disk 14 andoutput window 11 above sump 12, however, remain essentially suspended atthat location until the negative voltage interval, at which time thenegatively-charged contaminant particles are caused to migrate throughfluid 13 toward output window 1 1. At the same time, positivelychargedcontaminant particles are caused to migrate toward disk 14 and betransported by motion of the disk until the voltage again drops to zero,at which time the contaminant particles start to reenter fluid 13. Thosepositively-charged contaminant particles released from the disk in sump12, especially those released below the bottom of output window 11, arecaught up in the motion of the fluid in the sump, and are removed byfiltration.

FIG. illustrates apparatus which may comprise dc source 60 of FIG. 3. Apair of brushes 61 and 62, rotating at substantially constant speed inthe direction indicated by the arrows, are energized with positive andnegative potentials, respectively, through circular contacts 64 and 65,respectively, from a high voltage dc power supply 63. A pair ofcommutator surfaces 66 and 67 provide output voltages to conductivecoating 24 on disk 14 and to housing 50, respectively, shown in FIG. 3.

From FIG. 5 it can be seen that, as brushes 61 and 62 rotate, brush 61connects contact 64 to commutator 66 to provide a positive potential atthe terminal and brush 62 connects contact 65 to commutator 67 toprovide a negative potential at the I terminal. After a 2% minuteinterval of such potentials, brush 61 terminates contact with commutator66 and brush 62 terminates contact with commutator 67, so that, during aone-half minute interval, both the i and I terminals produce zero volts.After the one-half minute interval, brush 61 contacts commutator 67 andbrush 62 contacts commutator 66, so that the i terminal produces anegative potential and the T- produces a positive potential. Afteranother 2% minute interval, both brushes again terminate contact withboth commutators, so that both the i and 1 output terminals againproduce zero voltage for one-half minute. The cycle is then repeated.

FIG. 6 represents another view of apparatus which may comprise dc source60 of FIG. 3, wherein llike numbers designate like elements. Asillustrated, a synchronous motor 68 drives a shaft 70 which rotatesbrushes 61 and 62 thereabout on different radii. As described inconjunction with FIG. 5, the output voltage produced is illustrated bythe waveform of FIG. 4.

The foregoing describes a method and apparatus for electrostaticallycleaning the interior surface of the output window and the front surfaceof the rotating disk in a light valve, rendering reversible anydegradation in performance due to particulate accumulation on opposingglass surfaces of the output window and rotating disk. The inventionprevents any significant quantities of contaminants in the fluid betweenthe output window and rotating disk from becoming electrostaticallyadhered to either of these surfaces, and permits removal of contaminantparticles from a portion of the fluid situated within a narrowconfinement in communication with a larger quantity of the fluid.

While only certain preferred features of the invention have been shownby way of illustration, many modiflcations and changes will occur tothose skilled in the art. It is, therefore, to be understood that theappended claims are intended to cover all such modifications and changesas fall within the true spirit of the invention.

I claim:

1. In a light valve containing a rotatable disk on which a layer oflight-modulating fluid is carried, a portion of said disk beingsubmerged in a reservoir of said fluid, said light valve including anoutput window partially submerged in said reservoir such that a portionof said fluid is contained between said disk and said output window,apparatus for removing contaminants from said fluid comprising:

means for applying a dc voltage of cyclically reversing polarity acrosssaid portion of said fluid, said voltage being of sufflciently highamplitude electrostatically to move particulate matter away from saiddisk and said output window; and means for withdrawing fluid containingsaid particulate matter from said reservoir and replenishing saidreservoir with fluid in which the content of particulate matter has beenminimized. 2. The apparatus of claim 1 wherein said last-named meanscomprises:

fluid filtering means; and means circulating fluid through saidreservoir and said filtering means so as to remove said particulatematter from fluid withdrawn from said reservoir. 3. In a light valvecontaining a rotatable disk having one surface covered with aconductive, transparent coating on which a layer of light-modulatingfluid is carried, a portion of said disk being submerged in a reservoirof said fluid, said light valve including an output window partiallysubmerged in said reservoir such that a portion of said fluid iscontained between said disk and said output window, apparatus forremoving contaminants from said fluid comprising:

a conductive surface situated outside said reservoir; means coupling adc voltage of cyclically reversing polarity from said conductivetransparent coating to said conductive surface such that a dc voltageappears across said portion of said fluid with sufficiently highamplitude electrostatically to move particulate matter away from saiddisk and said output window; and means for withdrawing fluid containingsaid particulate matter from said reservoir and replenishing saidreservoir with fluid in which the content of particulate matter has beenminimized. 4. The apparatus of claim 3 wherein said last-named meanscomprises:

fluid filtering means; and means circulating fluid through saidreservoir and said filtering means so as to remove said particulatematter from fluid withdrawn from said reservoir. 5. In a light valvecontaining a rotatable disk having one surface covered with a conductivecoating on which a layer of light-modulating fluid is carried, a portionof said disk being submerged in a reservoir of said fluid, said lightvalve including an output window partially submerged in said reservoirsuch that a portion of said fluid is contained between said disk andsaid output window, apparatus for removing contaminants from said fluidcomprising:

a conductive surface situated outside said reservoir; a voltage sourceapplying an electrical potential of one polarity to said conductivecoating and an electrical potential of opposite polarity to saidconductive surface, said electrical potentials electrostatically movingparticulate matter through said fluid between said disk and saidconductive surface; means coupled to said voltage source for cyclicallyreversing polarity of said electrical potentials according to apredetermined duty cycle so as to reverse the directions of movement ofsaid particulate matter through said fluid; and means for withdrawingfluid containing said particulate matter from said reservoir andreplenishing said reservoir with fluid in which the content ofparticulate matter has been minimized.

6. The apparatus of claim wherein said means for cyclically reversingpolarity of said electrical potentials comprises first and secondmotor-driven brushes energized with positive and negative electricalpotentials, respectively, said brushes contacting first and secondcommutator surfaces, respectively, during a first interval andcontacting said second and first commutator surfaces, respectively,during a subsequent interval, said first commutator surface beingcoupled to said conductive coating and said second commutator surfacebeing coupled to said conductive surface.

7. The apparatus of claim 6 wherein said first and second commutatorsurfaces are spaced apart from each other so that, for a time betweensaid first interval and said subsequent intervals said first and secondbrushes apply no electrical potentials to said conductive coating and tosaid conductive surface by failure to contact either of said first andsecond commutator surfaces.

8. In a light valve containing a movable surface on which a layer oflight-modulating fluid is carried, apparatus for removing contaminantsfrom said fluid comprising:

means for applying a dc voltage of cyclically reversing polarity acrossa portion of said fluid, said voltage being of sufficiently highamplitude electrostatically to move particulate matter away fromsurfaces in contact with said fluid; and

means at a fixed location communicating with said layer oflight-modulating fluid for replacing fluid containing said particulatematter with fluid in which the content of particulate matter has beenminimized. 9. The apparatus of claim 8 wherein said last-named meanscomprises:

fluid filtering means; and means circulating said fluid containing saidparticulate matter through said filtering means so as to remove saidparticulate matter therefrom. 10. In a light valve containing a surfacecoated with a layer of light-modulating fluid supplied from a reservoirof said fluid, apparatus for removing contaminants from said fluidcomprising:

means for applying a dc voltage of cyclically reversing polarity acrossa portion of said fluid, said voltage being of sufficiently highamplitude electrostatically to move particulate matter away fromsurfaces in contact with said fluid so as to allow said particulatematter to exist in suspension in said fluid; and means for withdrawingfluid containing said particulate matter in suspension from saidreservoir and replenishing said reservoir with fluid in which thecontent of suspended particulate matter has been minimized. 11. Theapparatus of claim 10 wherein said lastnamed means comprises:

fluid filtering means; and means circulating fluid through saidreservoir and said filtering means so as to remove said particulatematter from fluid withdrawn from said reservoir.

1. In a light valve containing a rotatable disk on which a layer oflight-modulating fluid is carried, a portion of said disk beingsubmerged in a reservoir of said fluid, said light valve including anoutput window partially submerged in said reservoir such that a portionof said fluid is contained between said disk and said output window,apparatus for removing contaminants from said fluid comprising: meansfor applying a dc voltage of cyclically reversing polarity across saidportion of said fluid, said voltage being of sufficiently high amplitudeelectrostatically to move particulate matter away from said disk andsaid output window; and means for withdrawing fluid containing saidparticulate matter from said reservoir and replenishing said reservoirwith fluid in which the content of particulate matter has beenminimized.
 2. The apparatus of claim 1 wherein said last-named meanscomprises: fluid filtering means; and means circulating fluid throughsaid reservoir and said filtering means so as to remove said particulatematter from fluid withdrawn from said reservoir.
 3. In a light valvecontaining a rotatable disk having one surface covered with aconductive, transparent coating on which a layer of light-modulatingfluid is carried, a portion of said disk being submerged in a reservoirof said fluid, said light valve including an output window partiallysubmerged in said reservoir such that a portion of said fluid iscontained between said disk and said output window, apparatus forremoving contaminants from said fluid comprising: a conductive surfacesituated outside said reservoir; means coupling a dc voltage ofcyclically reversing polarity from said conductive transparent coatingto said conductive surface such that a dc voltage appears across saidportion of said fluid with sufficiently high amplitude electrostaticallyto move particulate matter away from said disk and said output window;and means for withdrawing fluid containing said particulate matter fromsaid reservoir and replenishing said reservoir with fluid in which thecontent of particulate matter has been minimized.
 4. The apparatus ofclaim 3 wherein said last-named means comprises: fluid filtering means;and means circulating fluid through said reservoir and said filteringmeans so as to remove said particulate matter from fluid withdrawn fromsaid reservoir.
 5. In a light valve containing a rotatable disk havingone surface covered with a conductive coating on which a layer oflight-modulating fluid is carried, a portion of said disk beingsubmerged in a reservoir of said fluid, said light valve including anoutput window partially submerged in said reservoir such that a portionof said fluid is contained between said disk and said output window,apparatus for removing contaminants from said fluid comprising: aconductive surface situated outside said reservoir; a voltage sourceapplying an electrical potential of one polarity to said conductivecoating and an electrical potential of opposite polarity to saidconductive surface, said electrical potentials electrostatically movingparticulate matter through said fluid between said disk and saidconductive surface; means coupled to said voltage source for cyclicallyreversing polarity of said electrical potentials according to apredetermined duty cycle so as to reverse the directions of movement ofsaid particulate matter through said fluid; and means for withdrawingfluid containing said particulate matter from said reservoir andreplenishing said reservoir with fluid in which the content ofparticulate matter has been minimized.
 6. The apparatus of claim 5wherein said means for cyclically reversing polarity of said electricalpotentials comprises first and second motor-driven brushes energizedwith positive and negative electrical potentials, respectively, saidbrushes contacting first and second commutator surfaces, respectively,during a first interval and contacting said second and first commutatorsurfaces, respectively, during a subsequent interval, said firstcommutator surface being coupled to said conductive coating and saidsecond commutator surface being coupled to said conductive surface. 7.The apparatus of claim 6 wherein said first and second commutatorsurfaces are spaced apart from each other so that, for a time betweensaid first interval and said subsequent intervals said first and secondbrushes apply no electrical potentials to said conductive coating and tosaid conductive surface by failure to contact either of said first andsecond commutator surfaces.
 8. In a light valve containing a movablesurface on which a layer of light-modulating fluid is carried, apparatusfor removing contaminants from said fluid comprising: means for applyinga dc voltage of cyclically reversing polarity across a portion of saidfluid, said voltage being of sufficiently high amplitudeelectrostatically to move particulate matter away from surfaces incontact with said fluid; and means at a fixed location communicatingwith said layer of light-modulating fluid for replacing fluid containingsaid particulate matter with fluid in which the content of particulatematter has been minimized.
 9. The apparatus of claim 8 wherein saidlast-named means comprises: fluid filtering means; and means circulatingsaid fluid containing said particulate matter through said filteringmeans so as to remove said particulate matter therefrom.
 10. In a lightvalve containing a surface coated with a layer of light-modulating fluidsupplied from a reservoir of said fluid, apparatus for removingcontaminants from said fluid comprising: means for applying a dc voltageof cyclically reversing polarity across a portion of said fluid, saidvoltage being of sufficiently high amplitude electrostatically to moveparticulate matter away from surfaces in contact with said fluid so asto allow said particulate matter to exist in suspension in said fluid;and means for withdrawing fluid containing said particulate matter insuspension from said reservoir and replenishing said reservoir withfluid in which the content of suspended particulate matter has beenminimized.
 11. The apparatus of claim 10 wherein said last-named meanscomprises: fluid filtering means; and means circulating fluid throughsaid reservoir and said filtering means so as to remove said particulatematter from fluid withdrawn from said reservoir.